1. Field of the Invention
The present invention relates to the transfer of data between a system using one type of interface to an external storage system using a different type of interface. More particularly, the invention relates to a converter that allows an existing extended-count-key-data storage system to be used as a storage device for a SCSI-type interfaced open host system.
2. Description of the Related Art
A direct access storage device (DASD) comprises one or more movably mounted disks coated with remnant magnetic material. Either fixed block or variable length records are recorded along circular track extents formatted on the disks. The tracks are concentric such that a movable arm with a READ or WRITE head or a switchable set of fixed heads must be positioned or activated over the desired track to effectuate transfer of the records to or from the disk. In an assembly of disks, the set of all tracks that can be accessed without repositioning the access mechanism is termed a "cylinder". Where a DASD stores 500 tracks per disk, there would be 500 cylinders. The cylinders may be grouped in logical or arbitrary sets. Contiguous cylinders are grouped together to form "bands" such that cylinders 0-10 might be a first band, and 11-100 a second band. Efficient data movement frequently has required bulk transfer, that is, staging a track's worth of data or even a cylinder's worth of data at a time, to another level of storage.
There is no connection between the logical organization of data such as datasets and their counterpart cylinder and track location in DASD storage. For some purposes, such as reducing READ data transfer time, it is advantageous to store the dataset records in contiguous tracks or cylinders. For other purposes, such as batch DASD recordation of random WRITE updates of records, the writing might be to contiguous DASD storage so that each record written would be remote from other records in the same dataset stored elsewhere in DASD.
Another level of storage common to DASD environments is virtual storage (VS). A VS device may comprise a single DASD using multiple disks or several DASDs where each employs one or more disks. Regardless, the allocation of data to the VS is coordinated by a special controller contained in the VS device or a host system/subsystem to which the VS system is connected. An implementation of VS could comprise a system using count-key-data (CKD) or extended-count-key-data (ECKD). The controller generally determines where data will be stored. For example, the controller decides which disk of the VS device will receive the data. By using a separate controller to allocate data to the various disks, precious host processor time is freed up for other purposes. As far as the host processor is concerned, the VS device appears to be one disk. One example of such an arrangement, hereafter referred to as a virtual storage system (VSS), is where the VS device or system is coupled to an ECKD system such as an IBM 3990 system--manufactured by the assignee of the current invention.
Commonly, the interface between a VSS system and a host system to which it may be connected is uniform. For example, a host system using a SCSI-type interface internally will usually be connected to a VSS system using a SCSI-type interface. However, certain types of conventional high end data processing equipment, deemed enterprise, may use a different channel-to-control-unit input/output (I/O) interface using fiber or optical cables as a transmission medium. This type of enterprise system connection, deemed "ESCON," may be found, for example, on a RAMAC multiple disk array. ESCON may also be used in connecting an MVS system to or within the VSS.
However, an ESCON interface used by the VSS precludes the use of VSS with "open" systems. An open system is a system whose characteristics comply with standards made available throughout the industry--such as SCSI-types, IDE, or EIDE--and that therefore cannot be connected to other systems not complying with the same standard. Simply put, it currently is not possible to connect an open host system utilizing an open interface with a VSS. What is needed is an apparatus and method that would allow a high end VSS to be attached to a host open system, thereby allowing the VSS to act as extended "open" storage for the open host system.